Chemical Deaeration in Boilers

Chemical deaeration is the use of Chemicals to remove the dissolved gases, usually oxygen.  Chemical deaeration is usually used after Mechanical deaeration.  Even after mechanical deaeration, all the oxygen will not be removed.  An chemical known as an oxygen scavenger is used.  This ensures that all the oxygen has been removed. 

A common oxygen scavenger is Sodium Sulphite.  Sodium sulphite reacts with the trace amounts of oxygen.  Sodium sulphite, however, cannot be used at high pressure as it can decompose to acidic gases which can increase corrosion. 

Another oxygen scavenger is hydrazine.  Hydrazine reacts with oxygen and produces volatile compounds which do not dissolve.  Hydrazine also does not cause corrosion.  However, the downside is that it is a carcinogen (cancer causing substance) and thus has to be used very carefully.  It may be banned in the future. 

Draught in Boilers

Draught or Draft refers to the pressure difference between the burner and the atmosphere.  This pressure difference or draught causes the air to flow from the burner to the atmosphere.  The residue of combustion such as waste gases, soot, etc are carried away by the flow of air.

Draught also has a great role to play in combustion.  The flow of fresh air into the burners is necessary for proper combustion.  Hence, the draught system should be designed such that the combustion can take place properly. 

The draught of a combustion system can be measured using a manometer when the furnace is in operation.  One end of the manometer is connected to the furnace while the other end is left open to the atmosphere.  The pressure difference indicates the draught of the system. 

There are different types of draught

  • Natural Draught where the draught occurs naturally due to the pressure difference between the furnace and the atmosphere. 
  • Induced Draught where the draught occurs by means of fans which create a negative pressure in the furnace causing fresh air to enter
  • Forced Draught where the draught occurs due to fans which provide combustion air and create a positive draught in the furnace.  This  drives the air through the chimney

Package Boilers

A Package Boiler is a boiler that has its own burner.  It is small in capacity and is fired by gas or oil.  It has a short start-up time.  It is able to generate a large quantity of steam at high pressure and temperature.

The Package Boiler can be used for different applications.  In some cases, the package boiler is used to generate steam for industrial applications.  The package boiler can also be used for power generation when used with a steam turbine. 

In some cases, the package boiler is used to supplement the output of another large boiler as a peak load source.  In some cases, the boiler can also be used as an emergency generator. 

Carbon Dioxide in Boiler Water

Carbon Dioxide is a constituent of air.  As such it get dissolved in water.  Carbondioxide mixes with steam to form carbonic acid.  Carbonic acid is an unstable compound.  It has a tendency to react with steel and can thus corrode piping.

Another way carbon is present in the water is in the form of bicarbonates.  These carbonates decompose in the boiler to produce carbon dioxide.   This carbon dioxide is usually present in the condensate. 

Carbon dioxide reduces the pH of the water.  This turns the water acidic which results in further corrosion. 

Hence, carbon dioxide has to be removed from the water.  One simple way of removing carbondioxide is by heating the water.  Heating the water reduces the solubility and thus removes the gas.  The water should be externally treated to remove the carbonates. 

Venting at specific locations of condensation can also reduce the carbon dioxide in the system. 

Insulation in Boilers

Insulation in Boiler system is a critical piece of equipment.  It is factor which has a direct relationship with efficiency and cost.  Heat Energy is produced in the boiler.  This energy is transferred using steam as a medium throughout the system.

Poor or damaged insulation can also result in condensate formation which can cause problems such as hammering.

The Insulation in Boiler systems has three main functions

  • To prevent heat leakage from the boiler and the system
  • To protect personnel from accidental contact with hot surfaces.
  • To ensure a tolerable working temperature in the boiler room and surrounding for the operating personnel

There are many different materials used for Boiler Insulation. Glass Wool is a common material. Other insulation Materials are Styrofoam, fibreglass and polystyrene.

Surface Blowdown in Boiler

Surface Blowdown in boiler is carried out to remove dissolved substances at the surface of the water.  That is, it is used to remove impurities which are in the liquid or dissolved phase.  Impurities which are in solid phase precipitate to the bottom where they are removed by the bottom blowdown.

The impurities and dissolved substances tend to form a layer of foam on the surface of the water.  This layer of foam needs to be removed to reduce the level of the dissolved substances. 

Since the blowdown involves removing water from the surface, it is called surface blowdown.
Surface blowdown is done by a pipe which is made to float a few inches below the water surface.  The pipe is connected to the outlet by means of a swivel joint.  The pipe can thus freely float in the water.  The pipe is held afloat by means of a float.

The pipe has a needle valve at its end.  The size of the valve opening can be adjusted based on the frequency and amount of blowdown required during each session.  Today, Automatic blowdown controllers which can control the rate and volume of the blowdown are also available. 

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